Long-term endoprosthesis

ABSTRACT

A long-term endoprosthesis in which a relatively large part of the surface, which in the implanted condition makes contact with the bone of the wearer of the implant, is provided with electrodes. These electrodes are connected with a source of a low frequency electrical voltage and are so arranged that they alternate with relatively short spacings in the contact zone. The electrode spacing preferably decreases in size in accordance with the size of the mechanical area loading on the corresponding part of the contact area, that is to say, the greater the loading, the smaller the spacing.

This is a division of application Ser. No. 779,127, filed Mar. 18, 1977.

BACKGROUND OF THE INVENTION THE PRIOR ART

The U.S. Pat. Nos. 3,387,880, 3,820,534 and 3,918,440 and thecorresponding UK patent specifications Nos. 1,311,519, 1,393,701 and1,393,703 describe arrangements for the healing of compound bonefractures, comminuted fractures in conjunction with damage to soft partsof the body, pathological fractures, spontaneous fractures and forovercoming difficulties in healing due to pseudoarthroses,osteomyelitides and the like, in the case of which a magnetic field isinduced to produce bioelectric potentials and currents in the defectivetissue and as a result to accelerate regeneration and to enable healingto take place in cases which are resistant to therapy.

For this treatment a coil is implanted in the vicinity of the fracturewhich represents the secondary inductance of a transformer and it isassociated with an osteosynthetic part of a conventional type as forexample a support plate, a pin driven into the marrow of the bone oreven a joint prosthesis, or a coil can be incorporated in such anosteosynthetic part of a joint prosthesis. The inducing magnetic fieldis excited in a coil surrounding the diseased tissue region. The coil issupplied from a low frequency generator with current and it constitutesthe primary inductance of the transformer. The low frequency magneticfield of the coil pervades the whole tissue region, which comprises thebone defect and induces an electrical voltage in the implanted coil, thevoltage being caused to act in the tissue via electrodes in the form ofbone screws, wires or via metallic foil arranged at the surface of animplant in an insulated manner.

Experiments on animals and numerous clinical applications of thistechnique show that in the tissue pervaded by the magnetic field thebone formation prompted by the low frequency alternating electricpotentials begins in the immediate neighborhood of the electrodes andcontinues in the tissue region between the electrodes. As has been shownby histological analysis, the stimulated growth of the bone follows thegradients of the electric potential. If the electrodes extend throughthe gap of a fracture, generally perpendicular to the fracture surfaces,"bone bridges" form under the influence of the induced potentials andthey can quickly take over the supporting function of the osteosyntheticelements. As soon as the load carrying capacity of the newly formed boneis sufficient, the whole osteosynthetic part can be removed from thetissue together with the implanted coil and the electrodes.

For bone fracture treatment and bone regeneration experience has shownthat a few discrete electrodes, for example two to four electrodes canbe used which owing to locally concentrated electric potentialsstimulate growth which starts and accelerates regeneration of thetissue. The purpose of the electrodes is a formation of a load carryingbone bridge where the bone is defective. The further conduct ofregeneration and consolidation of the bone then takes place under theinfluence of functional loading in accordance with Wolff's Law.

As regards an implant which is to remain in the tissue for extendedperiods of time, if possible for the whole of the life of the patient,the requirements applying are in many ways different and they are ineffect very much more exacting; the implant is to take over thefunctions of missing bone components, the support and bearing of anartificial joint or the anchoring of a tooth in the alveolus of the jawbone.

Metallic implants have not been found suitable for this purpose, sincethe metal is attacked and not accepted by the tissue. Furthermoreceramic materials developed in more recent times for implantationpurposes on the basis of aluminum oxide, quartz, silicon nitride andceramic compound materials still leave much to be desired. While theyhave a sufficient mechanical strength and there is a freedom from anysubstantial attack by body fluids ceramic implant materials have notsuccessfully overcome the problem, as was the case of previouslyemployed metallic implants, of the still insufficient degree ofbiological tolerance. Even in the case of experiments on animals theprocess of embedding of, for example, a ceramic joint prosthesis lastsapproximately three months despite the high regeneration capabilities ofanimals. During this time the joint must not be used, that is to say theextremity must be kept still. In the case of accident casualities oraged persons the implant does not become embedded or only does so in asubstantially longer period of time. However even periods of one quarterof a year in which the patient must keep the fracture still lead tosubstantial additional health problems.

Attempts have been made to shorten these times in which the patient mustkeep still by a special shaping of the implants, for example by the useof a threaded or support rib design, which anchors the implant in thebone with or without the use of a cement. While implants of this typeare capable of taking up loads sooner, a functional contact of theimplant with the vital tissue is restricted in the case of ceramicprosthesis designed upon these lines to the times applying for metalprosthesis, that is to say on average to approximately 6 to 7 years. Aninsiduous progressive encapsulation by connective tissue of the implantwhich is experienced as a foreign body thus leads in the course of timeto a painful loosening of the prosthesis. It then becomes necessary toremove the prosthesis and carry out a new implantation, whose lifeexpectancy will, however, be substantially reduced. With the speedyincrease in degenerative and accident damage to the human skeleton,which has to be repaired by the implantation of bone prosthesis (at arate of presently approximately 1500 cases daily) a medicinal andeconomic problem has been developing on a substantial scale.

SHORT SUMMARY OF THE INVENTION

One aim of the present invention is that of providing an endoprosthesiswhich is suitable as a long-term implant and whose anchoring in the boneis substantially more reliable and longer lasting than with previouslyproposed endoprosthesis.

This aim is achieved in accordance with the invention by the measuresclaimed in claim 1.

The subordinate claims relate to further developments and advantageousforms of long-term endoprosthesis.

In contrast to the above mentioned "electro"-osteosynthetic implants ofthe prior art, which only remain in the body tissue for the purpose ofthe growth of new bone until the damaged part of the bone can be loadedagain, the endoprosthesis in accordance with the invention have beenspecifically developed in order to provide a functional, forcetransmitting and trouble-free bond between the vital bone and thesurface of the prosthesis body as rapidly as possible and then to ensurethat it lasts for the rest of the patient's life. As has been found tobe particularly significant a relatively even biologically activecontact extending over a wide area, is produced and maintained betweenthe bone and the prosthesis structure.

LIST OF SEVERAL VIEWS OF THE DRAWINGS

In what follows embodiments of the invention will be described in detailwith reference to the accompanying drawings with the explanation offurther features, properties and advantages of the long-termendoprosthesis of the invention.

FIG. 1 is a side view of a hip joint head prosthesis in accordance withone embodiment of the invention.

FIG. 2A shows a cross-section in a plane A--A of FIG. 1.

FIG. 2B shows a longitudinal section in a plane B--B of FIGS. 1 and 2A.

FIG. 2C shows a view, corresponding to FIG. 2B, of a modifiedembodiment.

FIG. 3 shows a diagrammatic view of the shank part of an endoprosthesisin accordance with a further embodiment of the invention.

FIG. 3A shows a circuit diagram of a winding and electrode arrangementfor the embodiment in accordance with FIG. 3.

FIG. 4 shows a part view of a shank part of an endoprosthesis of thetype shown in FIG. 3.

FIG. 5 shows a view, corresponding to FIG. 4, with a somewhat modifiedcoil and electrode arrangement.

FIG. 6 shows a partly sectioned elevation of a hip joint prosthesis inaccordance with one embodiment of the invention.

FIGS. 6A and 6B show diagrammatic views of receiving coils and electrodearrangements for the hip joint prosthesis in accordance with FIG. 6.

FIG. 7 shows diagrammatically another hip joint prosthesis in accordancewith the invention.

FIGS. 7A and 7B show plan view of electrode arrangements for a hip jointprosthesis of the type shown in FIG. 7.

FIG. 7C shows a circuit arrangement for electrode arrangements asrepresented for example in FIGS. 6A, 7A and 7B.

FIGS. 8A and 8B show diagrammatically the manner of operation of hipjoint prosthesis of the type shown in FIGS. 6 and 7.

FIG. 9 shows a partly sectioned side view of a further hip jointprosthesis in accordance with the invention.

FIGS. 10A and 10B show preferred electrode arrangements.

FIG. 11 shows a partly sectioned side view of a tooth prosthesis inaccordance with the invention.

FIG. 12 shows an elevation of a further embodiment of a tooth prosthesisin accordance with the invention.

Description of the several embodiments of the invention

The long-term endoprosthesis in accordance with the invention comprisepreferably and at least on the surface a tissue-compatible ceramicmaterial as for example on the basis of aluminum oxide, silicon dioxideand/or silicon nitride. Furthermore compound materials of metallic ornon-metallic filaments can be employed in a non-metallic or metallicmatrix.

An important feature of the present long-term endoprosthesis resides inthat a relatively large part of the surface, which in the implantedcondition makes contact with the bone of the wearer of the implant, isprovided with electrodes. The electrodes are preferably disposedthroughout at least 50 percent of the surface area of contact. Theseelectrodes are connected with a source of a low frequency electricalvoltage and are so arranged that they alternate with relatively shortspacings in the contact zone. The electrode spacing preferably decreasesin size in accordance with the size of the mechanical area loading onthe corresponding part of the contact area, that is to say, the greaterthe loading, the smaller the spacing. The electrode spacing has amaximum of 10 mm.

The alternating voltage for the electrode can be induced in aconventional manner by an external magnetic alternating field. Since inthe case of a long-term implant in accordance with the invention afterembedding only relatively small electrical voltages are required inorder to keep the bone in contact with the implant "alive" and to ensurea firm seating of the prosthesis, in the present long-term implantmeasures are preferably taken in order to produce the necessary lowfrequency alternating voltages automatically during normal use of theendoprosthesis. This offers the advantage that no special treatment witha magnetic field generator is required. Preferably however, in additionto the internal alternating voltage generator, as a particularlyconvenient feature, as a part of this internal alternating voltagegenerator, a receiving coil is provided, in which a voltage can beinduced from outside, so that for the embedding or growing-in period ahigher alternating voltage can be caused to act at the electrodes. Forkeeping the tissue vital around the implantate structure it has beenfound in accordance with the invention that voltages are sufficientwhich produce average field strengths in the order of magnitude of 1.0to 10 mV/cm.

In FIGS. 1, 2A and 2B as a first embodiment of the invention a hip jointhead prosthesis is represented. It can for example consist oftissue-compatible aluminum oxide ceramic material and has a shank 10,which is mounted in a suitably truncated femur 12. The shank 10 isadjoined by an abutment surface 14 and a conventional joint head part16.

The shank 10 is provided externally with axially extending ribs 18,between which groove-like depressions 20 are provided. Furthermore theshank 10 has a cavity 22, in which a cylindrical coil 24 is located,which is wound on a tubular coil core 26.

On the surface of the shank there are two electrodes 28 and 30, whichrespectively have a comb-like row of strip or finger-like projections,which are so arranged that the projections in the two electrodesalternate along the periphery of the shank. The cavity 22 is accessibleby a cap 32, which is connected by a layer 34, produced by brazing, oftissue-compatible metal with the main part of the shank 10. The layer ofbrazing metal or spelter is insulated or made of the same metal, forexample osteosynthetic metal, as the electrodes, so that no galvanicelement is produced. The spelter layer 34 forms a part of the electrode28 and is connected with the strip-like projections 28a of thiselectrode, which in the case of the embodiment actually shown arelocated on the ribs 18. The projections 28a can be produced bydeposition from vacuum, metallization, electroplating or any other knownceramic metallization method and preferably extend somewhat into theside walls of the depressions 20 so that a continuous electricalconnection is ensured even if a part of the metal layer located on onerib 18 should be frictionally worn away or otherwise damaged on drivingthe shank 10 into the bone. The projections 30a of the electrode 30 alsoconsist of metallic layers and are located on the bottoms of thedepressions 20. They are either individually, or as shown in FIG. 2B, bymeans of an annular metallized zone connected via holes 36, filled withelectrically conducting material, with the interior of the cavity 22. Atthis position the ends of the cylindrical pick-up coil 24 make contactvia a spring 38 or, respectively, a metal part 40 arranged at the end ofthe coil core 26, with the electrodes 30 and 28 respectively.

In the cylindrical pick-up coil 24 it is possible, as is indicated inthe initially mentioned publications, to induce a low frequencyelectrical voltage, using an external coil supplied with AC and theinduced voltage is effective at the electrodes 28 and 30 and promotesthe embedding or "growing-in" of the shank 10 into the femur 12 andlater prevents devitalization of the bone structure adjacent to theshank of the prosthesis.

For the automatic production of a weak alternating voltage between theelectrodes 28 and 30 it is possible, as indicated in FIG. 2C, to arrangein the coil core 26 a rod-shaped permanent magnet 42 engaged by tworelatively weak helical springs 44 and 46 and arranged so that they canmove axially. In the axial direction of the cylindrical pick-up coil 24the turn density varies or, as is indicated in FIG. 2C, only occupies apart of the range of movement of the permanent magnet 42. Owing to theinertial forces which come into play on walking and other movements ofthe wearer of the prosthesis the permanent magnet 42 performs axialmovements in the coil core 26 and such movements automatically producethe desired low alternating voltage in the cylindrical pick-up coil 24.The system comprising the springs and the magnet should preferably havean inherent frequency of not more than 20 Hz.

FIG. 3 shows diagrammatically a shank 110 of an endoprosthesis, forexample of a joint prosthesis, which like the shank 10 (FIG. 1) canconsist of a tissue-compatible ceramic material. The ceramic shank 110comprises an embedded rod-like core 111 of ferromagnetic material with alow coercive force as for example of a "magnetically weak" ferrite. Onthe substantially cylindrical outer surface of the shank 110 there is acoil deposited from vapor, whose turns simultaneously serve aselectrodes. The coils can form a two-start screw and be electricallyconnected in series, as is represented in FIG. 3A, so that between twoadjacent coils a voltage difference in accordance with the voltage ispresent, which is induced in one of the two helical coil parts connectedin series. The clearance between turns of the coil 124 formed in thismanner can, as represented, be uneven in order to produce higher fieldstrengths in the part where there is a greater pressure loading betweenthe bone 112 and the prosthesis shank than in zones where there is alower loading.

FIG. 4 shows a part of a prosthesis shank 210, which has embeddedwindings 224 and embedded or deposited electrodes 228, which arerespectively connected with corresponding turns of the winding 224. Theouter side of the shank 210 can, as represented, be constructed as ascrew. If the electrodes are embedded the ceramic material lying betweenthe embedded electrodes and the surface of the shank is preferably madesemi-conducting by the implantation of doping materials (see for exampleGerman patent specification (Offenlegungsschrift) No. 1,939,267). Thearrangement of electrodes in different planes has the advantage, as isthe case with the embodiment of FIGS. 1 and 2, that radial voltagegradients are produced, which promote the growth of axially aligned bonestructure.

As is shown in FIG. 5 in the implant shank annular grooves can beprovided, in which disc windings 324, connected in series, areaccommodated. One respective coil of each disc winding is connectedelectrically with an externally placed electrode 328, 330 etc.

A radial field gradient can also be produced by providing two electrodeswith a radial spacing and which are separated by a relativelycoarse-pored layer of ceramic material. The surface of this layer thentherefore carries one electrode, for example in the form of a perforatedlayer, while the other electrode is located at the bottoms of the pores.Such a structure can be produced by applying, for example by sinteringon, on a ceramic body firstly a perforated but coherent layer ofmetallic particles and then a perforated layer of ceramic in such amanner that a part of the metallic particles is accessible through theholes and the ceramic material produces by virtue of the holes in themetallic layer a firm connection with the underlying ceramic material.The surface of the porous layer is then provided with a metallic layerforming the second electrode.

FIG. 6 shows a hip joint prosthesis, which consists of a ceramic partspherical cap 410 and a joint socket 411 of ceramic material. Theceramic part spherical cap 410 is mounted on a femur neck process 412.The joint socket 411 is implanted in the hip bone 413. In the ceramicpart spherical cap 410 and the joint socket 411 rod-shaped permanentmagnets 442 are arranged together with mutually opposite coils 424,which comprise rod-shaped magnetic cores 443 of magnetically softmaterial. In each case one respective permanent magnet 442 lies oppositeto a coil 424 with a softly magnetic core 443. The ends of the coils 424are respectively connected with electrodes 428 and, respectively, 430(FIG. 6A). One such electrode arrangement is respectively arranged onthe inner side, which is in contact with the femur neck process 412, ofthe ceramic part spherical cap 410 or, respectively, with the opposite,which is in contact with the bone 413, of the joint socket 411. In thecase of the embodiment as shown in FIG. 6 the ceramic part spherical cap410 comprises two coils 424; these coils can be connected in series orrespectively connected with their own electrode sets. The electrodes 428and 430 form two spirals which are interleaved and are electricallyinsulated from each other. The spacing between the spirals can beselected in accordance with the load distribution as has already beenexplained with reference to FIG. 3.

Instead of two separate electrodes it is also possible to use one singlespiral winding, as is represented in FIG. 6B. The return conductor 429intersecting the turns of the spiral is, just as is the case with thereturn conductor of the coils in accordance with FIG. 3A, insulated fromthe turns in a suitable manner, for example by an intermediate layerdeposited from vapor of silicon dioxide or by the construction as an"entrenched" conductor. In the case of FIG. 6B the voltage between theindividual turns of the spiral is effective.

If during the use of the hip joint prosthesis in accordance with FIG. 6the permanent magnets are displaced with respect to the coils, themagnetic flux extending through the coils is changed and as a result acorresponding voltage is induced in the coils, which becomes effectiveat the electrodes and this prevents devitalization (encapsulation) ofthe zone areas which are in contact with the prosthesis parts.

The hip joint prosthesis in accordance with FIG. 7 has a hip head part510, whose shank can be so constructed as already explained withreference to FIGS. 1 to 2C. The joint head part 516 in this case howevercomprises several radially arranged rod-shaped permanent magnets 542 andin the joint socket 511 there is a number of coils 524 embedded in sucha manner that the magnetic flux produced by the ferromagnets in thecoils changes during natural movement of the joint and as a result acorresponding voltage is produced in the coils. One permanent magnet canbe provided with several coils as is represented by the coils 524a and524b. The coils can naturally comprise magnetic cores with a highpermeability and a low coercive force as is the case with the embodimentin accordance with FIG. 6.

The outer side of the joint socket 511 is again provided with electrodeswhich are supplied by the coils. The electrodes 528 and 530 can haveprojections, which, as is shown in FIG. 7A, extend radially and areinterleaved in an alternating manner. As FIG. 7B shows the electrodescan also have the shape of eccentrically interfitting rings in the caseof which adjacent electrodes are connected with different coil ends. Inthe area with the greatest mechanical loading the electrode spacing isat a minimum.

The low frequency alternating voltage, which is produced by a receivingcoil and is supplied to an electrode arrangement, can have imposed on ita low direct voltage of for example a few tenths of a volt. This can becarried out as is shown for example in FIG. 7C, by arranging a parallelcircuit arrangement of a semi-conductor diode, as for example agermanium diode 529 and a capacitor 531 in the lead extending from areceiving coil 525 to an electrode system as for example the electrodesystem 528, 530 in accordance with FIG. 7A or an electrode system asshown in FIG. 6A or FIG. 7B. The receiving coil 525 is so dimensioned,taking into account the threshold voltage of the semi-conductor diode,that at the electrode system a direct voltage of, at the most, a fewtenths of a volt is produced and the capacitor 531 is so dimensionedthat at the electrodes the desired alternating voltage of a few tenthsof a volt results.

FIGS. 8A and 8B show diagrammatically how the voltage of the electrodescan be produced. The hip joint head part 616 comprises a radiallyarranged rod-shaped permanent magnet 642. The joint socket 611 comprisesa cylindrical pickup coil 624 with a radial axis, about which a magneticcore 643 with a high permeability is wound. The ends of the coil areconnected with electrodes 628 and 630 respectively. On movement of thejoint with respect to the joint socket conditions alternate in which thepermanent magnet 642 is directly opposite to the coil 624 (FIG. 8A) and,respectively, in which between the permanent magnet and the coil arelatively large spacing is present (FIG. 8B). The magnetic fluxextending through the coil varies accordingly between a high and a lowvalue and as a result a corresponding voltage is induced in the coil,which via the surface electrodes is applied at the tissue.

FIG. 9 shows an embodiment of a hip joint prosthesis in accordance withthe invention with a hip head part 716 and a joint socket part 711. Bothconsist of ceramic material, in which granules of magnetically soft andmagnetically hard material are embedded. The joint head is provided witha vapor deposited or embedded winding 724a, which is connected viaembedded conductors 725 with electrodes 728 and 730, which can beconstructed in accordance with FIG. 1.

The outer side of the joint socket 711 carries a winding 724b, which isjoined together at its ends. The granules can have a diameter of forexample 0.5 to 1 mm and are sintered in the ceramic material. Themagnetically hard granules are magnetized by a magnetic field pulse,preferably in the axial direction of the joint head part (arrow 445). Onmovement of the joint head with respect to the joint socket in this caseas well voltages are induced in turns. However it is possible to inducevoltages in the turns also using an external magnetic alternating fieldwith a high efficiency, since the magnetically soft granules concentratethe field and operate as a magnetic core for the windings.

FIG. 10A shows the electrical field distribution of a windingarrangement, which comprises two "bifilar" winding parts lying atdifferent levels and which in a manner similar to the arrangement shownin FIG. 3A, are connected in series with a coil winding. The distance Xbetween the surfaces, in which the winding parts are placed, does not inpractice need to be very large and it can for example be equal to halfor two thirds of the turn pitch or it could, as a further possibility,only comprise a few microns.

FIG. 10B shows a corresponding winding arrangement with windings, whichhave a rectangular profile. The windings, electrodes and other metalparts coming into contact with the tissue can for example be made of aplatinum-iridium alloy.

FIG. 11 shows a tooth prosthesis 810 of ceramic material which comprisesan embedded magnetic core 843 of high permeability and low coerciveforce. The root part 816 of the tooth prosthesis, which is embedded inthe jaw bone bears on its outer surface a coil winding 824, which can beconstructed like the coils described with reference to FIGS. 3, 3A, 4and 5 and FIGS. 10A and 10B and simultaneously serves as an electrode.

FIG. 12 shows a tooth prosthesis 910, in which a cylindrical pick-upcoil 924 is embedded with a soft magnetic core 943. The ends of thecylindrical pick-up coil 943 are connected, in a manner similar to thatshown in FIGS. 1 and 2 with comb-like interfitting electrodes 928 and930 electrically.

If in the upper and lower jaws two mutually opposite or at leastapproximately mutually opposite dental prosthesis are to be provided,they can be equipped, in a manner similar to the hip joint prosthesis inaccordance with FIGS. 6 to 8, with permanent magnet coil combinationsfor producing a voltage.

It is possible to produce the voltages necessary for the electrodes alsoby a piezoelectric effect. Thus for example a part of the implant, whichduring use is subjected to alternating loads, can be made ofpiezoelectric ceramic material or it can be provided with apiezoelectric element, which is electrically coupled with the electrodesand supplies the latter with an electric voltage changing with themechanical load. Thus for example in the connection between the jointhead part 516 and the shank part 510 a disc 517 of piezoelectric ceramicmaterial can be arranged and fixed in position by brazing and thevoltages produced on loading can be conducted away by a suitableinsulated conductor system and supplied to the electrode system of theshank.

The ribs 18 of the prosthesis shank 10 (FIGS. 1 and 2) can also have atriangular, chevron-like cross-section.

I claim:
 1. A long-term endoprosthesis comprising, in combination, abody member for implanting in the bone of a person, said body memberhaving an outer surface configured for intimate contact with the bone inwhich said body member is implanted, a conductor wound on said bodymember having turns disposed in closely spaced relationship within themajor portion of the area of contact between said body member outersurface and the bone, said conductor having opposite ends connectedtogether and means for coupling said conductor to a low frequencymagnetic field to produce an electromotive force in said wound conductorresulting in a voltage difference between the turns of said woundconductor, at least some of the turns of said wound conductor beingadapted to form electrodes for enhancing the growth of new bone in saidarea of contact.
 2. A long-term endoprosthesis in accordance with claim1 wherein said wound conductor comprises two interleaved helically woundturns arranged in tandum with the same direction of turns of asubstantially bifilar construction.
 3. A long-term endoprosthesis inaccordance with claim 1 wherein said wound conductor is in the form of aspiral.
 4. A long-term endoprosthesis in accordance with claim 1 whereinthe turns of said wound conductor are disposed throughout at least 50percent of said area of contact.
 5. A long-term endoprosthesis inaccordance with claim 1 wherein the spacing between the turns of saidwound conductor has a maximum of 10 mm.